A base pair change:

Options

A. 1 always causes amino acid replacement
B. 2. always alters protein function
C. 3. results in new species
D. 4. does not necessarily change the phenotype

Answer

**Step-by-Step Solution:**

1. **Understanding Base Pair Changes**: A base pair change refers to a mutation in the DNA sequence where one nucleotide is replaced by another. This can occur due to various factors such as environmental influences or errors during DNA replication.

2. **Evaluating Option 1**: The first option states that a base pair change "always causes amino acid replacement." This is incorrect because not all mutations lead to a change in the amino acid sequence. Some mutations are silent, meaning they do not alter the amino acid due to the redundancy of the genetic code.

3. **Evaluating Option 2**: The second option claims that a base pair change "always alters a protein formation." This is also incorrect. While some mutations can affect protein formation, others may not have any significant impact on the protein's structure or function.

4. **Evaluating Option 3**: The third option suggests that a base pair change "results in a new species." This is misleading. A single base pair change is unlikely to result in the formation of a new species. Speciation typically involves multiple genetic changes over time, not just a single mutation.

5. **Evaluating Option 4**: The fourth option states that a base pair change "does not necessarily change the phenotype." This is the correct answer. Many mutations can occur without affecting the observable characteristics (phenotype) of an organism. This is particularly true for silent mutations or mutations that occur in non-coding regions of DNA.

6. **Conclusion**: After evaluating all the options, the correct answer is that a base pair change does not necessarily change the phenotype.